Lietard, Olivier, Schlumberger Dowell, Bellarby, Jonathan, BP Exploration Operating Co. Ltd., Holcomb, David, ProTechnics International Inc.
This paper discusses all phases of the stimulation of naturally fractured carbonate, oil reservoirs in the North Sea.
The considered reservoirs are located in the vicinity of salt domes. Most of them are in the central part of the UK sector, east of Aberdeen. The reservoir rock is the well-known Tor chalk, whose mechanical properties have been greatly modified by the proximity of the diapirs. The matrix rock itself is much harder (more similar to a lime-stone) and the whole reservoir is fissured, particularly on the top of the dome. Double-porosity behaviour is usual.
Wells drilled through these reservoirs suffer from very severe damage due to the invasion of the natural fractures by large amounts of drilling mud and loss circulation materials. Stimulation consists of high rate damage removal treatments, alternating stages of crosslinked gel, customised acid formulations and ball sealers. Pre and post job well performances demonstrate huge productivity index increases due to the clean-up of the natural fissures around the wellbore.